Our long-range goals are to uncover immunologically-a portion of the cell stagespecific changes in surface proteins associated with the maturation of murine erythroid colony-forming units (CFU-E), to isolate recombinant clones encoding some of these proteins from cDNA-expression libraries, and to use such clones to assess stage- specific gene-level events. Our assumption is that erythroid maturation provides an end-stage situation in which a substantial body of cell-stage specific events occur but, when set against the magnitude of change encountered in organismal development, are comparatively modest in number. In this potentially simplified setting, our intent is to establish a repertoire of often anonymous but differentially expressed genes with which we and others can hope to begin dissecting the ensemble of regulatory events underlying some aspects of erythroid maturation. Seven Specific Aims are set: first enlarge our existing collection of rat monoclonal antibodies (MoAb against murine cell-surface antigens preferentially expressed at a particular stage of erythroid maturation; second identify MoAb especially suitable for later use; third, supplement our existing reticulocyte globin-poor-cDNA expression library with a plus/minus one; fourth, by applying selected MoAb to the screening of expression libraries, isolate cDNA clones encoding preferentially expressed cell-surface elements; fifth, validate stage-specific expression of thus isolated cDNA; sixth, isolate genomic DNA sequences corresponding to such cDNA; and seventh, utilize cDNA clones and corresponding genomic sequences (a) to characterize specific gene structures and gene-level events associated with differential gene expression during erythroid maturation and (b) ultimately, identify those loci and their products which modulate changing expression of the genes in our repertoire.